Diazotization of o-tolidine

If the hydrochloric acid is added all at once instead of in two portions as described, a solid will be obtained consisting of o-tolidine coated with its dihydrochloride and the diazotization will occur slowly. 

In a i-l. beaker 21.2 g. (o.i mole) of d-tolidine is made into a thin paste with 300 cc. of water and dissolved by the addition of 20 cc. (23.6 g., 0.23 mole) of concentrated hydrochloric acid (sp. gr. 1.18), warming a little if necessary. The solution is cooled to 10° with ice, mechanical agitation is started, and 21 cc. (35 g., 0.24 mole) more of concentrated hydrochloric acid added (Note i). This causes partial separation of o-tolidine dihydrochloride. Keeping the temperature at 10-15°, a solution of 14.5 g. (0.2 mole) of gs per cent sodium nitrite in 40 cc. of water is run in rapidly, holding back the last 10 per cent which is added slowly and only as needed to give a distinct positive test for nitrous acid on starch-iodide paper. This excess is maintained for one-half hour. Throughout the diazotization an excess of hydrochloric acid should be present as shown by a strong test... 

B/s-diazotized benzidine has been used to create active-hydrogen-reactive spacer arms on chromatographic matrices (Silman et al., 1966 Lowe and Dean, 1971). The compound may be used similarly to o-tolidine for the conjugation of active-hydrogen-containing molecules (see Section 9.1, this chapter, and Chapter 2, Section 6.1). 

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